I have to go with the "classics".
Ray Bradbury tops my list I think. Even though I have barely scratched the surface of all the man has to offer, I enjoy everything I have read.
H.G. Wells - Hard to beat one of the fathers of Science-fiction.
I really don't have any favorite modern authors... OH! I'd count David Gerrold. Love his ability to mix in a little fun into a good story.
Recently read Terry Pratchett's "Mort". Liked it OK, but couldn't really get into "Carpe Jugulum".
"Too Soon from the Cave, Too Far from the Stars"